The nonadiabaticity question for europium(III/II): Outer-sphere reactivities of europium(III/II) cryptates

The one-electron reduction kinetics of the europium cryptates Eu(2.2.1)³⁺ and Eu(2.2.2)³⁺ by the aquo ions V_aq ²⁺ and Eu_aq ²⁺ and the oxidation kinetics of Eu(2.2.1)²⁺ by Co(NH₃)₆ ³⁺ have been studied by using a polarographic technique in order to examine the effects of encapsulating europium within cryptate cavities upon the reactivity of the Eu(III/II) couple. At 25°C and an ionic strength μ = 0.1, the second-order rate constants (M^-1 s^-1) for acid-independent pathways are as follows: Eu(2.2.1)³⁺-V_aq ²⁺, 0.5; Eu(2.2.1)³⁺-Eu_aq ²⁺, ca. 0.2; Eu(2.2.2)³⁺-Eu_aq ²⁺, 1.5; Eu(2.2.2)³⁺-Eu_aq ²⁺, 1.4; Co(NH₃)₆ ³⁺-Eu(2.2.1)²⁺, 0.055. By comparison of these kinetic data with those for similar reactions involving the Eu_aq ^3+/2+ couple, the rate constant for Eu(III/II) self-exchange, k_ex, is estimated to increase by factors of ca. 1 × 10⁷ and 2 × 10⁴ upon encapsulation of europium in (2.2.1) and (2.2.2) cryptate cavities, respectively. Estimates of k_ex equal to ca. 10, 4 × 10^-2, and 5 × 10^-6 M^-1 s^-1 (μ = 0.1) for Eu(2.2.1)^3+/2+, Eu(2.2.2)^3+/2+, and Eu_aq ^3+/2+, respectively, are obtained from the Marcus cross relation. The increases in k_ex resulting from cryptate encapsulation suggest that nonadiabaticity is not primarily responsible for the extremely low reactivity of Eu_aq ^3+/2+. The values of k_ex are shown to be roughly consistent with the Franck-Condon barriers estimated from structural data.